A servo control device that drives a mechanical device such as a machine tool generates a drive command to a motor that drives a tool and a table by feedback control, so that positions of the tool and the table provided in a machine follow a command value. As a motor that drives the tool and the table, there is a case in which a rotary motor is used or a case in which a linear motor is used. A drive command becomes a torque command when the rotary motor is used, and becomes a propulsion command when the linear motor is used. In the present specification, a case in which the rotary motor is used is explained.
Particularly, servo control that drives a mechanical system in a mechanical device so that a tool position with respect to an object to be machined (a workpiece) accurately follows a commanded path (commanded locus) is referred to as “locus control” or “contour motion control”, and the servo control is performed precisely by using a numerical control device and a servo control device attached thereto. The mechanical system in a mechanical device that is to be controlled has a plurality of shafts, and motors constituting those shafts are controlled to drive by using the servo control device.
The servo control device also has a function of predicting and correcting an error (particularly, an error in locus) generated due to a disturbance in addition to the feedback control, in order to suppress an influence of a disturbance factor such as friction present in the mechanical system. Particularly, when the moving direction of a feed shaft is reversed, the direction in which the friction acts is also reversed, and the influence thereof appears conspicuously in the locus error.
As a typical example thereof, there is a tracking error that occurs when a circular arc locus is commanded and the moving direction of the feed shaft is reversed in a quadrant switching portion of the circular arc locus. When an error amount in a radial direction is enlarged and plotted, this error has a shape in which the locus projects outward in a protruding shape, and thus the error is referred to as “quadrant protrusion”. If a tracking error such as the quadrant protrusion occurs, slits or flaws are generated in the machining result, which is not desirable.
Therefore, for example, conventionally as shown in Patent Literature 1, there has been a method of estimating a change in friction torque generated before and after a motion direction of a feed drive mechanism is reversed, at the time when a position command direction is reversed, and adding the estimated change amount of the friction torque to a torque command.